1. Field of the Invention
The present invention relates to a laminated extruded resin sheet comprising a methyl methacrylate resin, particularly, a laminated extruded resin sheet comprising a methyl methacrylate resin which is subjected to secondary thermoforming to obtain a formed article having smaller bias of thickness.
2. Description of the Related Art
A methyl methacrylate resin is a resin having excellent transparency and light resistance, and widely used in an optical material, lighting cover, illumination signboard and the like.
This methyl methacrylate resin is also excellent in secondary thermoforming property, and can be endowed with specific form by heating at temperature over heat deformation temperature and draw forming, after extruding once to form a sheet.
Examples of this secondary thermoforming method include free blow forming, free vacuum forming, pushing up forming, ridge forming, straight forming, drape forming, reverse draw forming, air slip forming, plug assist forming, plug assist reverse draw forming and the like, these forming methods are used alone or in combination to obtain desired form.
Recently, a lighting cover having corner portion extremely extruding and deep bath tub come to produce by super draw forming, and the forms of molded articles come increasingly further complicated by progress in forming technology itself.
High draw forming with short heating time or heating at low temperature also come to carry out as the development of the forming processing technology, therefore, resins to be formed are also required to be designed so as to cope with the severe conditions in this forming.
When the above-mentioned high draw forming is conducted using a conventional methyl methacrylate resin, there is a tendency that a sheet is broken in forming, or difference in thickness between high draw part and low draw part of a sheet remarkably increases even if forming is possible.
For solving these problems, there are disclosed an extruded sheet to which a fine particle having specific branched structures is added, and an extruded sheet obtained by allowing a methyl methacrylate polymer having specific cross-linked structure and a rubber-like polymer to be contained in a methyl methacrylate resin, for example, in Japanese Patent Application Laid-Open (JP-A) No. 9-208,789.
The extruded sheet described in JP-A 9-208789 is excellent, however, since it is liable to be contracted in effecting secondary thermoforming, there is a tendency that a uniform thin sheet is not easily obtained unless cooling condition is successfully controlled in producing an extruded sheet. Therefore, there is a problem that slight difference in forming property occurs between a sheet produced in summer and a sheet produced in winter in an extrusion production factory of which temperature control is difficult. Further, this sheet is expensive since effective ingredients are dispersed in the whole plate.
Then, the present inventors have intensively studied an extruded resin sheet comprising a methyl methacrylate resin which produced without specific conditioning in extrusion molding, and as a result, found that there is obtained a laminated extruded resin sheet comprising a methyl methacrylate resin which is produced without being influenced by cooling condition in producing an extruded sheet and can be subjected to secondary thermoforming to obtain a molded article having smaller bias of thickness, only by laminating thin layers of a methyl methacrylate resin in which a insoluble specific acrylic resin particle is dispersed onto both surfaces of a sheet of a methyl methacrylate resin in which the specific amount of a rubber-like polymer is dispersed, completing the present invention.